50 research outputs found
OFDM demodulation in underwater time-reversed shortned channels
This work addresses the problem of OFDM transmission
in dispersive underwater channels where impulse responses
lasting tens of miliseconds cannot be reliably handled
by recently proposed methods due to limitations of channel
estimation algorithms. The proposed approach relies on passive
time reversal for multichannel combining of observed waveforms
at an array of sensors prior to OFDM processing, which produces
an equivalent channel with a shorter impulse response that
can be handled much more easily. A method for tracking
the narrowband residual phase variations of the channel after
Doppler preprocessing is proposed. This is a variation of an
existing technique that can improve the spectral efficiency of
OFDM by reducing the need for pilot symbols. This work also
examines techniques to handle sparse impulse responses and
proposes a channel estimation method where an l1 norm is
added to the standard least-squares cost function to transparently
induce sparseness in the vector of channel coefficients. Algorithms
are assessed using data collected during the UAB’07 experiment,
which was conducted in Trondheim fjord, Norway, in September
2007. Data were transmitted with bandwidths of 1.5 and 4.5 kHz,
and recorded at a range of about 800 m in a 16-hydrophone array.
Significant multipath was observed over a period of at least 30
ms.FC
Chirp-rate quasi-orthogonality based DSSS-CDMA system for underwater acoustic channel
Abstract(#br)Underwater network node positioning is a key supporting technology for underwater networks. Generally, nodes at known locations (anchor nodes) are used to transmit location information to the node to be located, and the node to be located performs location calculation according to the arrival time of the received information, which requires multiple access communication between the anchor nodes and the node to be located. In order to reduce the multiple access interference (MAI) and distinguish the information of different anchor nodes, it is necessary to study the underwater multiple access method. The GPS positioning system uses direct sequence spread spectrum code division multiple access(DSSS-CDMA) technology, and the positioning satellite transmits a signal formed by the BPSK modulation by transmitting a pseudo-random sequence (PN sequence) to the binary code sequence corresponding to the navigation message to the user equipment to be located. The underwater acoustic (UWA) channel has a complex multipath structure and Doppler effect, which causes a large interference to the communication system. Compared with narrowband communication, spread spectrum communication has a strong anti-interference ability, and can maintain the reliability of the communication system in the UWA channel. Based on this, this paper proposes a quasi-orthogonal Chirp-rate based DSSS-CDMA method under UWA channel, and carries out simulation analysis and experimental verification
Vector Approximate Message Passing based Channel Estimation for MIMO-OFDM Underwater Acoustic Communications
Accurate channel estimation is critical to the performance of orthogonal
frequency-division multiplexing (OFDM) underwater acoustic (UWA)
communications, especially under multiple-input multiple-output (MIMO)
scenarios. In this paper, we explore Vector Approximate Message Passing (VAMP)
coupled with Expected Maximum (EM) to obtain channel estimation (CE) for MIMO
OFDM UWA communications. The EM-VAMP-CE scheme is developed by employing a
Bernoulli-Gaussian (BG) prior distribution for the channel impulse response,
and hyperparameters of the BG prior distribution are learned via the EM
algorithm. Performance of the EM-VAMP-CE is evaluated through both synthesized
data and real data collected in two at-sea UWA communication experiments. It is
shown the EM-VAMP-CE achieves better performance-complexity tradeoff compared
with existing channel estimation methods.Comment: Journal:IEEE Journal of Oceanic Engineering(Date of
Submission:2022-06-25
Spread Spectrum Modulation with Grassmannian Constellations for Mobile Multiple Access Underwater Acoustic Channels
The objective of this study is to evaluate Grassmannian constellations combined with a spread spectrum multiple access scheme for underwater acoustic mobile multiple access communication systems. These communication systems enable the coordination of a fleet of Autonomous Underwater Vehicles (AUVs) from a surface or bottom control unit, e.g., a boat. Due to its robustness against phase rotation, the demodulator of Grassmannian constellations uses non-coherent detection, and the main advantage of such modulation lies in the spectrum efficiency gain with respect to conventional differential modulation. The communication system under study in this paper consists of (i), at the transmitter side, a Grassmannian modulation used in an orthogonal spread spectrum multiple access scheme called Multiuser Hyperbolic Frequency Modulation (MU-HFM) and (ii), at the receiver side, a non-coherent array decoder. The modulation and demodulation are presented as well as the considered spreading sequences. Finally, performances of the proposed transmission scheme are evaluated over replayed underwater acoustic channel responses collected at sea by a multi-sensor acoustic acquisition system.Spread Spectrum Modulation with Grassmannian Constellations for Mobile Multiple Access Underwater Acoustic ChannelspublishedVersio
Effects of errorless learning on the acquisition of velopharyngeal movement control
Session 1pSC - Speech Communication: Cross-Linguistic Studies of Speech Sound Learning of the Languages of Hong Kong (Poster Session)The implicit motor learning literature suggests a benefit for learning if errors are minimized during practice. This study investigated whether the same principle holds for learning velopharyngeal movement control. Normal speaking participants learned to produce hypernasal speech in either an errorless learning condition (in which the possibility for errors was limited) or an errorful learning condition (in which the possibility for errors was not limited). Nasality level of the participants’ speech was measured by nasometer and reflected by nasalance scores (in %). Errorless learners practiced producing hypernasal speech with a threshold nasalance score of 10% at the beginning, which gradually increased to a threshold of 50% at the end. The same set of threshold targets were presented to errorful learners but in a reversed order. Errors were defined by the proportion of speech with a nasalance score below the threshold. The results showed that, relative to errorful learners, errorless learners displayed fewer errors (50.7% vs. 17.7%) and a higher mean nasalance score (31.3% vs. 46.7%) during the acquisition phase. Furthermore, errorless learners outperformed errorful learners in both retention and novel transfer tests. Acknowledgment: Supported by The University of Hong Kong Strategic Research Theme for Sciences of Learning © 2012 Acoustical Society of Americapublished_or_final_versio
Design of energy-efficient ultrasonic communication systems on steel pipes
Ultrasonic communication provides an alternative to radio-frequency (RF) by transmitting guided ultrasonic signals along installed or buried metallic pipes. Buried pipe corrosion monitoring and intermittent infrastructure data collection are potential application areas, for which reliable wireless links are unavailable, due to strong RF attenuation in soil, or through shielded building infrastructure. When designing a network of such links, energy efficiency, defined as the average energy per transmitted bit, can be far more important than Shannon capacity, for such battery-powered, relatively inaccessible links. This work focuses on the low-rate, total life-time energy-limited regime to maximize battery life, while maintaining reliable information transfer at a nominal average rate. The strong frequency selectivity of the through-pipe ultrasonic channel poses several challenges for low-power systems, including strong intersymbol interference (ISI). Previous works have suggested a variety of ad hoc design schemes to implement low-power communication systems satisfying minimum data rate requirements under highly frequency selective and lossy conditions, but failed to propose a systematic methodology to optimize design parameters for energy efficiency. In this work, we apply the concept of energy efficiency maximization to ultrasonic communication over steel pipe channels. A cross-layer approach accounting for both transmit power and signal processing power is suggested, where frequency division multiplexing is explored to counter frequency selectivity. Finally, bits-per-Joule capacity of this channel, based on experimentally measured channel responses, are determined numerically, and an example of an optimized multi-tone frequency shift keying (MFSK) scheme is suggested